Prior art window materials, such as glass and plastic, have high transmissivity for electromagnetic energy of a wide range of frequencies, including visible light, IR and RF energy. In some applications, for example stealthy military aircraft, a window's ability to pass radio frequency energy is a disadvantage. Not only do traditional materials fail to absorb RF energy, but they also permit reflections from behind the window to pass back through the window.
This issue has been addressed in some applications by placing a metal screen behind the window, thereby making it reflective of RF energy. The screen reflects the RF energy while allowing visible or IR energy to pass. By careful shaping of the reflective screen it is at least possible to direct the reflected RF energy in a desired direction; e.g. away from a hostile radar installation. Disadvantages of this approach include the attenuation of the visible and IR energy passing through the window/screen, and the optical distortion which can be caused by the screen material. Alternatively, a thin gold film may be applied to the window to make it reflective of RF energy while still transparent to visible/IR energy. This approach is a partial solution at best. A preferred approach would be to have a window which absorbs RF radiation while transmitting visible light and/or infrared radiation without significant distortion.
Electromagnetic wave absorbers have been the subject of years of research and development, as chronicled in "Electromagnetic Wave Absorbers and Anechoic Chambers Through the Years" by William H. Emerson, published in IEEE Transactions on Antennas and Propagation, Volume AP-21, No. 4, July 1973, pages 484-490. One well known type of RF energy absorber, known as the Jaumann absorber, consists of resistive sheets separated by low loss foam spacers placed in front of a conductor of infinite conductivity, as described more fully in "Tables for the Design of Jaumann Microwave Absorber" by J. R. Nortier, et al., published in Microwave Journal, September 1987, pages 219-222, incorporated by reference herein. Prior art electromagnetic wave absorbers are not useful in applications requiring windows, since the absorbers are opaque to visible and infrared energy.